Combustion systems are widely used by industry to provide heat to different substrates, such as steel, aluminum, cement, and the like. These load materials require considerable energy to undergo chemical and physical changes that are required to transform the load materials into more useful forms. Combustion systems typically require an oxidant in combination with a fuel to generate the large amount of energy needed to carry out chemical and physical transformation of the load materials. Typically, a hydrocarbon fuel is mixed with air or oxygen to release the combustion energy. During operation, the combustion systems generate fumes that take away some of the energy introduced by the combustion fuel. The fumes represent an energy loss mechanism that removes energy that otherwise should have been transferred into heating the load material. In this manner, substantial losses of energy can occur that impairs the efficiency of the combustion system and leads to energy waste. To reduce the energy loss, heat recovery systems are used that capture the heat of the flue gases and transfer it to another medium to perform useful work, as mechanical energy, electrical energy, chemical energy, and the like.
To improve the efficiency of the combustion system, the waste heat can be transferred back into the combustion fuel. Heat recovery systems are known that combine several solutions to enhance the efficiency of a combustion system. See for example, U.S. Pat. No. 4,492,568 to Palz and U.S. Pat. No. 4,475,340 to Tseng. In addition to heating the combustion fuel, systems are known in which the efficiency is improved by preheating the load material. For example, in the glass industry, a cullet preheating system on an oxygen-fuel combustion furnace transfers flue gases through a raining bed of cullet or batch pellets that are heated before entering the combustion furnace. See for example, U.S. Pat. No. 5,578,102 to Alexander and U.S. Pat. No. 5,526,580 to Zippe. Although the technique of preheating raw materials increases the combustion efficiency, such techniques are difficult to implement because of the extensive apparatus needed for handling large, bulky raw materials. The handling problems make such systems difficult to retrofit into existing combustion systems. Further, the engineering modifications necessary for installation of the heat recovery equipment can make the systems very expensive to build.
The preheating of natural gas is known technology for most combustion applications using heat recovery. It can be achieved through heat exchangers that recover the heat from the flue gases. Systems described in the U.S. Pat. Nos. 4,492,568 and 4,475,340 are applied in both combustion engines and industrial furnaces. These systems involve metallic parts that conduct the heat between the natural gas and the flue gases, and usually preheat the natural gas to temperatures below about 400.degree. C. In heat recovery systems used to preheat natural gas, it is very important that structurally defective metallic components of the heat exchangers not be exposed to highly reducing conditions at elevated temperatures. The disassociated carbon from the natural gas can easily diffuse into structural defects, such as weld joints. The diffusion of the disassociated carbon can cause carburizing effects in the metal, and lead to case hardening and micro-crack formation in the welded joints.
To avoid potentially dangerous conditions arising from the formation of cracks in heat exchanger materials, heat exchangers can be built using non-metallic components. For example, a ceramic heat exchanger is described in U.S. Pat. No. 5,630,470 to Lockwood. Although avoiding the use of welded metals, materials such as ceramics are often fragile both mechanically and thermally, and they can fail in an unpredictable manner. In an environment where the heat transfer fluids may undergo abrupt temperature variations due to process settings, any failure of the ceramic material can trigger massive combustion in the heat recovery system. The potential danger associated with ceramic heat exchangers is shared by heat exchangers employing other materials, such as plastics and reinforced plastics, and the like. For example, U.S. Pat. No. 5,323,849 to Korezynski describes a corrosion resistant heat exchanger in which materials are selected for their corrosion and erosion resistance. However, it is highly unlikely that heat exchangers employing ceramic and plastic materials can be safely operated for preheating an oxidizer in a fuel combustion system.
The direct exchange of heat between waste flue gases and oxidizers used in a combustion system presents engineering challenges in the design of a safe and efficient heat exchange system. The breakdown or down time of such a heat exchange system can cause serious process interruption and increase production costs. Accordingly, a need exists for a heat exchanger that can preheat highly combustible fuels, such as hydrocarbon fuels, and oxidizers, such as oxygen, and the like, and that can be operated safely and efficiently.